Support name and number accessors on enum values

[LINKIWI]

Currently, Protobuf enum values have a String() method defined, which provides a human-readable representation of the value with str(...) in Starlark, including the value's name and number (index). However, there is no interface to retrieve only the name and number of an enum value for use in Skycfg programs.

This PR proposes augmenting the enum value type with two attributes, name and number, which expose the enum value descriptor's name and number, respectively.

This enables a definition like:

enum Foo {
    HELLO = 0;
    WORLD = 1;
}

to be used in Skycfg like:

pb.Foo.HELLO.name  # "HELLO" (string)
pb.Foo.HELLO.number  # 0 (int)

I added unit tests to cover the new logic (as well as some additional tests to cover existing logic for the str(...) representation of values).

$ bazelisk test --cache_test_results=no //...
INFO: Invocation ID: 0c384b38-64b0-461b-985b-ed281b040175
INFO: Analyzed 14 targets (0 packages loaded, 1 target configured).
INFO: Found 8 targets and 6 test targets...
INFO: Elapsed time: 0.102s, Critical Path: 0.03s
INFO: 7 processes: 1 internal, 6 linux-sandbox.
INFO: Build completed successfully, 7 total actions
//:skycfg_test                                                           PASSED in 0.0s
//go/assertmodule:assertmodule_test                                      PASSED in 0.0s
//go/hashmodule:hashmodule_test                                          PASSED in 0.0s
//go/protomodule:protomodule_test                                        PASSED in 0.0s
//go/urlmodule:urlmodule_test                                            PASSED in 0.0s
//go/yamlmodule:yamlmodule_test                                          PASSED in 0.0s

Executed 6 out of 6 tests: 6 tests pass.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.